In general, Native Americans were used as proxies in the wars of empire that were fought between the British and the French during the eighteenth century.

In these wars (like the Seven Years War, which is better known in the US as the French and Indian War), Native Americans fought on both sides of the war. Typically, what would happen was that different Native American tribes would side with different imperial powers. For example, if Tribe A took the French side, their enemies, Side B, would take the British side. The Native Americans acted as scouts and as supplemental forces in these wars.

Agree completely with the post above, with a few additions. These proxy wars took part mainly between the French and British, with the Spaniards sometimes involved. The French had a distinct population disadvantage, having settled in the New World so late. Only about 70,000 French settlers were here by 1754, compared with a million British-Americans. This forced them to be both friendlier towards the Natives (so the French settlements and missions could continue to live here) and to ally with them when war came. Natives knew the land, and how to fight guerrilla wars, and this gave the French an early military advantage.

The British also used Native allies where possible, usually with tribes that were enemies with the tribes allied with the French (confused yet?). The end result being that after the French and Indian War, many Native tribes remained armed, and this intensified future resistance against the empires and later, the US.

There were other imperial conflicts in the New World at that time, but the French and Indian War is by far the largest and most significant.